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MINOR: [C++] Add mising pragma-once (#35657)

### Rationale for this change

The C compilation model.

### What changes are included in this PR?

Adding a `#pragma once` that I forgot to add when I created this header.

### Are these changes tested?

N/A.

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>